1. Field of the Invention
The present invention relates to an addition-curable silicone composition and an optical element, particularly, an addition-curable silicone composition excellent in rubber property and strength characteristic that provides a cured product having a low refractive index and an optical element encapsulated with the cured product of the composition.
2. Description of the Related Art
An addition-curable silicone composition comprises a polyorganosiloxane containing an aliphatic unsaturated group such as an alkenyl group, and provides a cured product obtained by curing the polyorganosiloxane from hydrosilylation reaction. The cured product thus obtained is excellent in heat resistance, cold resistance and electric insulation, and its high transparency is associated with use of each encapsulant for optical purposes.
A polyorganosiloxane composition used for optical purposes and an encapsulant for an optical element comprising the composition are required to meet properties of a high transparency and a high refractive index, and it is a common practice to use a copolymer of dimethylsiloxane and diphenylsiloxane or polymethylphenylsiloxane as a main backbone.
However, the above polysiloxane that provides a cured product having a high refractive index of 1.54 or more is difficult to synthesize. Meanwhile, a cured product of a branched polysiloxane which a phenyl group is introduced is capable of having a refractive index of 1.53 to 1.54, but a cured product obtained is in the form of a hard resin and less elastic. Under the circumstances, a composition composed of a branched organopolysiloxane and a linear organopolysiloxane is proposed, but the composition fails to meet a satisfactory transparency, refractive index and elasticity (Patent Documents 1 to 7).
Therefore, an addition-curable silicone composition that provides a cured product having a high transparency, a high refractive index and a satisfactory strength characteristic, and an encapsulant for an optical element comprising the composition are proposed in Patent Document 8, but the resulting performance of the optical element fails to reach sufficiently favorable level, and particularly, the optical transmission with a wavelength of 400 nm at 25° C. is unfavorable.